This invention relates to radio frequency receiving and transmitting antennas, and, more particularly, to receiving and transmitting radio frequency antennas for use in magnetic resonance imaging.
Magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d) is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.
MRI uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue.
During an MRI procedure, the patient is inserted into an imaging volume containing a static magnetic field. The vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. A transmitting antenna within the imaging volume emits a pulse or pulses of radio frequency energy having a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, shifting the vectors of the nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or xe2x80x9cprecessxe2x80x9d around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (xe2x80x9cMRxe2x80x9d) signals, at the resonant or Larmor frequency, which are detected by a radio frequency receiving antenna tuned to that frequency. The receiving antenna is typically positioned within the imaging volume proximate the patient. Gradient magnetic fields are provided to spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals detected by the receiving antenna are amplified, digitized and processed by the MRI system. The same antenna may act as the transmitting and receiving antenna. Hydrogen, nitrogen-14, phosphorous-31, carbon-13 and sodium-23 are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By known mathematical techniques involving correlation of the gradient, magnetic fields and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient, it is possible to determine the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient""s body. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest. Typical MRI systems are the Quad 7000 and Quad 12000 available from FONAR Corporation, Melville, N.Y. for example.
The quality of the magnetic resonance image is directly related to the characteristics of the receiving and transmitting antenna. Significant electrical characteristics of the antenna include its sensitivity, Q factor and the signal-to-noise ratio.
Sensitivity is the signal voltage generated in the receiving antenna by MR signals of a particular magnitude. The higher the sensitivity within the region to be imaged, the weaker the signals which can be detected. The sensitivity of the antenna is preferably substantially uniform with respect to MR signals emanating from all volume elements within the region of the subject which is to be imaged.
The Q or quality factor, which is closely related to the sensitivity of the antenna, is a measure of the ability of the antenna to amplify the received signal. The Q-value of the antenna can be lowered by a patient proximate or within an antenna, due to capacitive and to a lessor extent the inductive coupling between the patient and the antenna. Antennas must therefore have a high Q-value when they are unloaded and the Q-value must not become too diminished by the presence of the patient. On the other hand, the coil must couple well with the region of a patient""s anatomy which is to be imaged.
Signal-to-noise (xe2x80x9cS/Nxe2x80x9d) ratio is the ratio between those components in the electrical impulses appearing at the antenna terminals representing the detected MR signals and the components representing spurious electromagnetic signals in the environment and internally generated thermal noise from the patient. To optimize the S/N ratio, the antenna should have low sensitivity to signals from outside the region to be imaged and to thermal noise. To enhance both S/N ratio and sensitivity, the antenna is xe2x80x9ctunedxe2x80x9d or arranged to resonate electrically at the frequency of the MR signals to be received (the Larmor frequency), which is typically several megahertz or more. Neither the coil size nor geometry of the antenna can be allowed to create an inductance or self-capacitance which prevents tuning to the desired frequency.
The antenna must also meet certain physical requirements. The antenna should have a high filling factor, which maximizes the amount of tissue which fits within the volume detected by the windings of the coil. The antenna must also fit within the relatively small imaging volumes typically provided for receiving the subject within the magnet assembly, along with other components of the system and the subject. The antenna should not cause significant discomfort to the subject. Additionally, the antenna should be easy to position with respect to the subject, and be relatively insensitive to minor faults in positioning relative to the subject.
These numerous considerations often conflict with one another and therefore must be balanced during the design process.
The sensitivity and S/N ratio of MRI radio frequency antennas have been improved by positioning a first coil, tuned to resonate at the Larmor frequency of the element of interest, proximate the part of the subject which is to be imaged, and positioning a similarly tuned second coil, typically a single loop, adjacent to the first coil. The second coil is connected to the pre-amplifier of the MRI system. The first and second coils are inductively coupled to each other. MR signals emitted by the patient induce voltages in the first coil, causing current to flow within the coil. The current generates a magnetic field which induces voltage in the second coil. The MR signals may induce voltages in the first coil, as well. The voltages induced in the first coil are processed by the MRI system. Use of two such coils amplifies the MR signals and filters spurious signals outside of the frequency band of the Larmor frequency. See, for example, U.S. Pat. No. 5,583,438 and U.S. Pat. No. 5,575,287, assigned to the assignee of the present invention.
Radio frequency antenna coils may be used in a variety of configurations. For example, the coil may be receiving coil, as discussed above. The receiving coil may be part of an array of receiving coils, such as in the primary and secondary coil arrangements, also discussed above. The receiving coil may also act as the transmitting coil of the MRI system. A pair of receiving coils can also be arranged 90xc2x0 with respect to each other to enable quadrature detection, which improves the signal-to-noise ratio.
In accordance with the present invention, an MRI antenna is disclosed comprising a first coaxial cable unit comprising a first inner conductor with first and second ends and a first outer conductor substantially surrounding the inner conductor, the first outer conductor having first and second ends. The first coaxial cable unit has a concave curvature facing a first direction. A second coaxial cable unit is provided also comprising an inner conductor with first and second ends and an outer conductor substantially surrounding the inner conductor. The outer conductor has first and second ends, as well. The second coaxial cable unit has a concave curvature facing a second direction opposite the first direction. The concave curvature of the first and second coaxial cables generally face each other to define a region between the first and second coaxial cable units for receiving a body part. The first end of the first inner conductor may be electrically connected to the first end of the second inner conductor through a first capacitor and the first end of the first outer conductor may be electrically connected to the first end of the second outer conductor through a second capacitor. Preferably, the first ends of the inner conductors and the first ends of the outer conductors are on opposite sides of the first and second coaxial cable units. The second end of the first inner conductor and the second end of the second inner conductor are not connected to each other and the second end of the first outer conductor and the second end of the second outer conductor are not connected to each other. They are, however, preferably inductively coupled to each other and a distributed capacitance exists between them. While the circuits including the inner and outer conductors are open circuits, it is believed, without limiting the scope of the invention, that the inductive coupling and distributed capacitance between the inner and outer conductors of the first and second coaxial cable units enables the circuits to be tuned to the same frequency. The first and second outer conductors provide an output of the antenna. The inner and outer conductors of the first and second coaxial cable units are tunable to the same frequency.
In accordance with another embodiment of the present invention, an MRI antenna is disclosed comprising a base for surrounding a body part. The base has an upper portion supporting at least one upper coaxial cable unit and a lower portion supporting at least one lower coaxial cable unit. The coaxial cable units comprise an inner and an outer conductor substantially surrounding the inner conductor. The at least one upper coaxial cable unit has portions adjacent to the at least one lower coaxial cable unit for inductively coupling the upper and lower coaxial cable units during operation. The inner conductor of the at least one upper coaxial cable unit is capacitively connected to the inner conductor of the at least one lower coaxial cable unit through a single capacitive connection and the outer conductor of the at least one upper coaxial cable unit is capacitively connected to the outer conductor of the at least one lower coaxial cable unit through a single capacitive connection. An output of the antenna is provided from the outer conductors of the at least one upper coaxial cable unit and from the at least one lower coaxial cable unit.
In accordance with another embodiment of the present invention, an MRI antenna is disclosed comprising first, second and third coaxial cable units. Each coaxial cable unit comprises inner and outer conductors, wherein each outer conductor substantially surrounds each inner conductor. The inner and outer conductors of each coaxial cable unit are inductively coupled during operation. The first, second and third coaxial cable units lie in first, second and third parallel planes, respectively. The first plane is between the second and third planes and the first and third coaxial cable units are aligned. The first, second and third coaxial cable units have a concave curvature, wherein the curvature of the first and third coaxial cable units is in a first direction and the curvature of the second coaxial cable unit is in a second direction opposite the first direction. The first, second and third coaxial cable units define a region for receiving a body part. The second coaxial cable unit has portions adjacent to the first and third coaxial cable units for inductively coupling the second coaxial cable unit to the first and third coaxial cable units. The inner and outer conductors of the first and third coaxial cable units are electrically connected to form respective circuits tuned to the same frequency. The inner conductors of the first and third coaxial cable units are capacitively coupled to the inner conductor of the second coaxial cable unit through a single capacitive connection and the outer conductors of the first and third coaxial cable units are capacitively coupled to the outer conductor of the second coaxial cable unit through a single capacitive connection. The antenna further comprises outputs from the coupled outer conductors of the first and third coaxial cable units and from the outer conductor of the second coaxial cable unit.
In accordance with another embodiment of the present invention, an MRI antenna is disclosed comprising first, second, third, fourth and fifth coaxial cable units. Each coaxial cable unit comprises inner and outer conductors. The outer conductor of each coaxial cable unit substantially surrounds the inner conductor of the unit and the inner and outer conductors of each are inductively coupled to each other during operation. The first, second, third, fourth and fifth coaxial cable units lie in first, second, third, fourth and fifth parallel planes, respectively. The first plane is between the third and fourth planes and the second plane is between the fourth and fifth planes. The first and second coaxial cable units are aligned and the third, fourth and fifth coaxial cable units being aligned. The first, second, third, fourth and fifth coaxial cable units each have a concave curvature. The curvature of the first and second coaxial cable units is in a first direction and the curvature of the third, fourth and fifth coaxial cable units is in a second direction opposite the first direction. The first, second, third, fourth and fifth coaxial cable units define a region for receiving a body part. The first coaxial cable unit has portions adjacent to the third and fourth coaxial cable units for inductively coupling the first coaxial cable unit to the third and fourth coaxial cable units, during operation. The second coaxial cable unit has portions adjacent to the fourth and fifth coaxial cable units for inductively coupling the second coaxial cable unit with the fourth and fifth coaxial cable units during operation. The inner and outer conductors of the first and second coaxial cable units are electrically connected to form respective circuits tuned to a frequency and the inner and outer conductors of the third, fourth and fifth coaxial cable units are electrically connected to form respective circuits tuned to the frequency. The inner conductors of the first and third coaxial cable units are capacitively coupled to the inner conductors of the third, fourth and fifth coaxial cable units through a single capacitive connection and the outer conductors of the first and second coaxial cable units are capacitively coupled to the outer conductors of the third, fourth and fifth coaxial cable units through a single capacitive connection. An output from the antenna is provided from the outer conductors of the first and third coaxial cable units and from the outer conductors of the third, fourth and fifth coaxial cable units. The circuits including the respective inner and outer conductors are tunable to the same frequency. The inner and outer conductors of the third, fourth and fifth coaxial cable units are preferably connected in parallel to lower the inductance of the antenna array, enabling tuning to the high Larmor frequencies in high magnetic field strengths (6,000 Gauss and above). In addition, the capacitive connection from the inner and outer conductors of the first and second coaxial cable units to the respective inner and outer conductors of the third, fourth and fifth coaxial cable units, minimizes the generation of eddy currents. This embodiment is particularly suited for magnetic resonance imaging of the lumbar spine.
Transmitting antennas and magnetic resonance imaging systems including the antennas of the present invention are also disclosed.